Personally I think you're being massively unreasonable. But the thought of going on holiday with my brother's girlfriend doesn't fill me with joy, and if that announcement came after she lived with us for a year I'd be devastated. In saying that, I am an introvert, so I'd find it incredibly draining to constantly feel like I had to be "on" all the time, since there was someone other than family in my home. Is your DD an introvert by any chance? It may help explain why she's feeling the way she is.
This past year has been tough for everyone, but at least your son has had his girlfriend around all the time to keep him company. Your DD on the other hand, is probably missing her friends, and classmates (presuming she's still in school), and on top of that has had to put up with her brother's girlfriend moving in, probably unexpectedly due to the pandemic. They might get on okay but the girlfriend may not be someone she would have met or become friends with in other circumstances. They're friendly because of your son's relationship, and because they've been living in close quarters for months now. Your DD wasn't the one who chose to date her, or live with her, and she's probably fed up of an extra person around the house. I imagine she hasn't said anything because she doesn't want to upset anyone else in the family, but the holiday situation has tipped her over the edge. Plus you said she misses hanging out with her brother, presumably because the girlfriend is always included when they spend time together at home.
She might have been looking forwards to this trip, not only because she viewed it as the last proper family holiday for the four of you, but also because she thought she'd finally get to spend some time alone with her brother, like she used to. And now everyone else in the family has unilaterally decided that the girlfriend gets to come on the family holiday - without consulting her. I imagine she's disappointed and upset, but also feeling excluded that she wasn't included in these conversations. Maybe she's wondering why it was discussed with everyone else but her? And maybe feeling unsure about her place within the family - if you feel like the GF is almost family, perhaps your DD feels like she's not needed anymore as there's another (almost) daughter. Combine that with uncertainty about the future, pandemic fatigue, and good old teenage hormones, she's maybe feeling insecure?
And having another person will totally change the dynamic, esp as it's a short city break. If it was the beach holiday, where people had more opportunity to do things by themselves (eg your DC hanging out together while the GF sunbathes alone) it wouldn't be so bad. But on a city break, where you've less time at the location, and presumably are planning to do activities together, there wouldn't be as much opportunity for your DC to hang out anyway. And this decision has scuppered any chances of that happening. Plus she'll be a third (fifth?) wheel now, so I can understand her not wanting to go anymore.
Also just because you consider the GF almost family, doesn't mean your DD does. If you eventually decide against taking the GF, a good get out could be you realising that both your children are growing up and moving on with their lives (DS in a relationship, DD finishing school). And you've realised that actually you'd like one last holiday with only your children, before everything changes and they properly fly the nest.